9 
an opening for the admission of cold air. By regulating the 
size of this opening, sufficient cold air lowers the temperature 
of the heated blast so that it can be read by means of the ordi- 
nary mercurial thermometer. By knowing the amount of di- 
lution by cold air, the temperature of the heated blast can be 
approximated. 
23. MESURE AND NOUEL’S OPTICAL PYROMETER. This pyrome- 
ter is based upon the refractive power of quartz for rays of light 
of different intensities. The angle of rotation is measured by 
two Nicols prisms, and serves as a measure of temperature. 
24. LE CHATELIER AND CORNU’S PHOTOMETRIC PYROMETER 
consists of a photometer so arranged to compare the intensity 
of the luminous rays of light projected from a standard lamp 
flame with the rays of light issuing from the furnace or other 
heated object. The degree of adjustment of the aperture 
through which the furnace rays pass in order to equalize the 
intensities of the rays of light serves to measure the tempera- 
ture. 
25. LE CHATELIER’S THERMO-ELECTRIC PYROMETER consists of 
two wires, forming the thermo couple, which produces the elec- 
tric current when introduced into a furnace or flame, the gal- 
vanometer, which measures the current produced ; and the wires 
connecting both. The intensity of the current produced is re- 
sultant upon the temperature of the thermo couple in the fur- 
nace, and thus serves to measure it. 
Nos. 20 to 25 are exhibited by J. Struthers. 
26. HELIOSTAT. A modification of the Foucault form; made 
by the Geneva Optical Co. Exhibited by Dr. William Stratford. 
27. MICROSCOPE STAGE. For photomicrography and measuring 
objects. Exhibited by Dr. William Stratford. 
28. RULING MACHINE. Invented by Dr. Irvin Sickels. Rules 
on metal or glass, micrometers or gratings, I to 24,000 lines 
to the inch. Exhibited by Dr. William Stratford. 
29. PANORAMA mounting of Rowland’s photographs of the grat- 
ing solar spectrum. Exhibited by W. Hallock. 
